A Bioinspired Iron‐Molybdenum μ‐Nitrido Complex and Its Reactivity toward Ammonia Formation

Construction and Function of Thiolate-Bridged Diiron NxHy Nitrogenase Model Complexes

ConspectusBiological nitrogen fixation mediated by nitrogenases has garnered significant research interest due to its critical importance to the development of efficient catalysts for mild ammonia synthesis. Although the active center of the most studied FeMo-nitrogenases has been determined to be a complicated [Fe7S9MoC] hetero-multinuclear metal-sulfur cluster known as the FeMo-cofactor, the exact binding site and reduction pathway of N2 remain a subject of debate. Over the past decades, the majority of studies have focused on mononuclear molybdenum or iron centers as potential reaction sites. In stark contrast, cooperative activation of N2 through bi- or multimetallic centers has been largely overlooked and underexplored, despite the renewed interest sparked by recent biochemical and computational studies. Consequently, constructing bioinspired bi- or multinuclear metallic model complexes presents an intriguing yet challenging prospect. In this Account, we detail our long-standing research on the design and synthesis of novel thiolate-bridged diiron complexes as nitrogenase models and their application to chemical simulations of potential biological N2 reduction pathways.Inspired by the structural and electronic features of the potential diiron active center in the belt region of the FeMo-cofactor, we have designed and synthesized a series of new thiolate-bridged diiron nitrogenase model complexes, wherein iron centers with +2 or +3 oxidation states are coordinated by Cp* as carbon-based donors and thiolate ligands as sulfur donors. Through the synergistic interaction between the two iron centers, unstable diazene (NH═NH) species can be trapped to generate the first example of a [Fe2S2]-type complex bearing a cis-μ-η1:η1-NH═NH subunit. Significantly, this species can not only catalyze the reductive N-N bond cleavage of hydrazine to ammonia but also trigger a stepwise reduction sequence NH═NH → [NH2-NH]- → [NH]2-(+NH3) → [NH2]- → NH3. Furthermore, an unprecedented thiolate-bridged diiron μ-nitride featuring a bent Fe-N-Fe moiety was successfully isolated and structurally characterized. Importantly, this diiron μ-nitride can undergo successive proton-coupled electron transfer processes to efficiently release ammonia in the presence of separate protons and electrons and can even be directly hydrogenated using H2 as a combination of protons and electrons for high-yield ammonia formation. Based on combined experimental and computational studies, we proposed two distinct reductive transformation sequences on the diiron centers, which involve a series of crucial NxHy intermediates. Moreover, we also achieved catalytic N2 reduction to silylamines with [Fe2S2]-type complexes by ligand modulation.Our bioinspired diiron cooperative scaffold may provide a suitable model for probing the potential N2 stepwise reduction pathways from the molecular level. Different from the traditional alternating and distal pathways dominated by mononuclear iron or molybdenum complexes, our proposed alternating transformation route based on the diiron centers may not involve the N2H4 intermediate, and the convergence point of the alternating and terminal pathways is imide, not amide. Our research strategy could inform the design and development of new types of bioinspired catalysts for mild and efficient nitrogen reduction in the future.

Bimetallic H2 Addition and Intramolecular Caryl-H Activation Mediated by an Iron-Zinc Hydride

Heteronuclear Fe(μ-H)Zn hydride Cp*Fe(1,2-Cy2PC6H4)HZnEt (3) undergoes reversible intramolecular Caryl-H reductive elimination through coupling of the cyclometalated phosphinoaryl ligand and the hydride, giving rise to a formal Fe(0)-Zn(II) species. Addition of CO intercepts this equilibrium, affording Cp*(Cy2PPh)(CO)Fe-ZnEt that features a dative Fe-Zn bond. Significantly, this system achieves bimetallic H2 addition, as demonstrated by the transformation of the monohydride Fe(μ-H)Zn to a deuterated dihydride Fe-(μ-D)2-Zn upon reaction with D2.

Nitrogen activation and surface charge regulation for enhancing the visible-light-driven N2 fixation over MoS2/UiO-66(SH)2

A series of dehydrated MoS2/UiO-66(SH)2 (MS/UiS) composites has been prepared as photocatalysts for N2 fixation. Typically, 10% MS/UiS exhibits the best performance with an NH4+ yield rate of 54.08 μmol∙g-1∙h-1. 15N isotope test confirmed that the sample 10% MS/UiS was most effective for reducing N2 to ammonia. Such enhanced activity was due to the presence of abundant unsaturated Zr and Mo sites which would synergistically promote the adsorption and activation of N2. The photogenerated electrons would transfer to the unsaturated Zr-O clusters while part of photogenerated electrons at the interface migrate to MS via MoVI-O interactions between MS and UiS. These two electron transfer pathways effectively promote the separation of photogenerated carriers. The activated N2 is reduced to ammonia by the synergistic effect of protonated hydrogen and photogenerated electrons. Finally, a possible N2 fixation mechanism is proposed which emphasizes the significant roles of nitrogen activation and interface interaction in composites photocatalyst for improving photocatalytic performance.

Unusual Hydrogenation Reactivities of a Thiolate-Bridged Dicobalt μ-Nitride Featuring a Bent {CoIII-N-CoIII} Core

Transition metal nitrides have received considerable attention owing to their crucial roles in nitrogen fixation and nitrogen atom transfer reactions. Compared to the early and middle transition metals, it is much more challenging to access late transition metal nitrides, especially cobalt in group 9. So far, only a handful of cobalt nitrides have been reported; consequently, their hydrogenation reactivity is largely unexplored. Herein, we present a structurally and spectroscopically well-characterized thiolate-bridged dicobalt μ-nitride [Cp*CoIII(μ-SAd)(μ-N)CoIIICp*] (2) featuring a bent {CoIII(μ-N)CoIII} core. Remarkably, complex 2 can realize not only direct hydrogenation of nitride to amide but also stepwise N-H bond formation from nitride to ammonia. Specifically, 2 can facilely activate dihydrogen (H2) at mild conditions to generate a dicobalt μ-amide [Cp*CoII(μ-SAd)(μ-NH2)CoIICp*] (4) via an unusual mechanism of two-electron oxidation of H2 as proposed by computational studies; in the presence of protons (H+) and electrons, nitride 2 can convert to dicobalt μ-imide [Cp*CoIII(μ-SAd)(μ-NH)CoIIICp*][BPh4] (3[BPh4]) and to CoIICoII μ-amide 4, and finally release ammonia. In contrast to 2, the only other structurally characterized dicobalt μ-nitride Na(THF)4{[(ketguan)CoIII(N3)]2(μ-N)} (ketguan = [(tBu2CN)C(NDipp)2]-, Dipp = 2,6-diisopropylphenyl) (e) that possesses a linear {CoIII(μ-N)CoIII} moiety cannot directly react with H2 or H+. Further in-depth electronic structure analyses shed light on how the varying geometries of the {CoIII(μ-N)CoIII} moieties in 2 and e, bent vs linear, impart their disparate reactivities.